A conventional semiconductor photodetector used in an optoelectronic telecommunication system utilizing optical fibers as a medium of communication. FIG. 3 exhibits an example of a prior photodetector device. The photodetector device has a photodiode chip (1) which converts light beams into photocurrents, a header (2) for holding the photodiode chip (1) thereupon, leads (3) for sending bias voltage or photocurrent to the photodiode chip (1), a cap (4) for covering the photodiode chip (1) with a transparent window (5), a lens (9) for converging light signals, a lens mount (10) for keeping the lens (9) in a coaxial position in front of the photodiode chip (1), a receptacle (8) having a cylindrical portion for sustaining a ferrule (7). The cap (4) and the transparent window (5) airtightly keep the photodiode chip (1) in the closed space enclosed by the header (2), the cap (4) and the transparent window (5). The ferrule (7) fixes an end of an optical fiber (6) for the transmission of light signals. The lens (9) is provided between the end of the optical fiber (6) and the photodiode (1) for converging light beams emitted from the optical fiber on the photodiode chip (1) efficiently. Such a photodetector device shown by FIG. 3 is called a coaxial type.
Conventional photodetector devices aim to a coupling of a photodiode chip with a single-mode fiber or a multi-mode fiber for transmitting light signals. For example, for the light of 1.3 .mu.m wavelength, a single-mode fiber has a core whose diameter is about 10 .mu.m and a multi-mode fiber has a core of about 50 .mu.m in diameter. The light beams emitted from the end of an optical fiber spread in a free space as a cone whose top angle is determined by the difference of refractive indices between a core and a cladding of the fiber. The lens (9) effectively converges the spreading light cone on a receiving region of the photodiode. When a high speed type photodiode with a narrow receiving region less than 100 .mu.m in diameter is used as a photodiode chip, a non-spherical lens or selfoc lens is preferably adopted as the lens, because the non-spherical lens and the selfoc lens have less aberration than an ordinary spherical lens. The adoption of such lens raises the cost of producing the photodetecting device.
Furthermore, even if such an expensive non-spherical or selfoc lens is used, the adjustment of positions of the fiber end, the lens and the receiving area of the photodiode requires rigorous preciseness. Spot welding by a YAG laser is used twice to weld the header (2) to the lens mount (10) along line A and to weld the lens mount (10) to the receptacle (8) along line B. Owing to twice adjustment processes and twice welding processes, some aberration is likely to occur at welding line B between the receptacle (8) and the lens mount (10) or at welding line A between the lens mount (10) and the header (2), which leads to a low yield of products between 70% and 80%. Moreover, it takes a long time to align three parts, i.e. the receiving region of the photodiode, the center of the lens and the end of the optical fiber, rigorously.
What we must avoid is that the light beams emitted from the end of the fiber irradiate on an exterior part (non-receiving region) of the photodiode. FIG. 4 demonstrates the condition that the light beams irradiate on the non-receiving region. There is actually a lens (9) between the fiber end and the photodiode chip (1) in a conventional one. A straight line simply denotes the lens. The lens will converge light beams (16) emitted from a fiber core (6') on a narrow receiving region of the photodiode chip as shown by dotted lines (16).
What would occur if the lens (9) were omitted? Since the beams would not be converged and would spread in a cone, some outer beams (20) would attain the exterior part (non-receiving region) of the photodiode. The photodiode chip (1) has an n-type or p-type semiconductor substrate (11), and epitaxial layers (12) with the same conduction property epitaxially grown on the substrate (11), a dish-like diffusion region (13) produced by diffusing some dopant at the center of the epitaxial layers (12). The diffusion region (13) has the reciprocal conduction property to the epitaxial layers (12). The boundary (18) between the epitaxial layers (12) and the diffusion region (13) is called a pn-junction (18). Depletion region, where electrons and holes are swept away by the electrical field, is formed at both side of the pn-junction (18). Thickness of the depletion region is some microns. An annular electrode (14) is formed on the diffusion region (13). Another electrode (15) is formed on the bottom surface of the semiconductor substrate (11).
The fiber (6) emits light beams bearing signals in a light cone defined by the difference of refractive indices between the core and the cladding. Most of the light beams are absorbed within the central receiving region on the pn-junction. The beams excite pairs of electrons and holes near the pn-junction. Since a strong electric field stands in the depletion region by the annular electrode (14) and the bottom electrode (15), the electrons and holes are accelerated by the strong electric field. The carriers (electrons and holes) run at high speed toward the epitaxial layers (12) and the diffusion region (13). The flow of the carriers produced by light is called a photocurrent. The photocurrent flows out through the the annular electrode (14) and the bottom electrode (15) and is amplified to some extent. Such beams (16) which enter the receiving region within the annular electrode (14) induce no problem.
However, other parts of beams (20) sometimes enter the non-receiving region outside the pn-junction (18). The extra beams (20) invite a serious problem. The extra beams (20) also excite pairs of electrons and holes. Since the non-receiving region lies outside the depletion region, there is no electric field in the non-receiving region. The electrons and holes produced in the non-receiving region diffuse in all directions. Some of the carriers trek toward the electrodes (14) and (15) at slow velocity. The others are lost by the recombination with majority carriers. Slow trekking carriers make a delayed photocurrent.
The delayed photocurrent will incur a delay of phase and deform analog waves in the case of detection of analog signals. The delayed part of photocurrents will deform pulse waves in the detection of digital signals. Especially, the delay will induce a long tail on the falling portion of a rectangular pulses as shown in FIG. 5. The duration of the tail is about microseconds. The tail of the reproduced pulse will disturb high speed digital communication. To avoid the occurrence of the delayed photocurrent, expensive converging optics must be used at present in order to converge the light beams from an optical fiber on a narrow receiving region of a photodiode chip without leak beams entering the non-receiving region outside the pn-junction (18). An optoelectronic communication system cannot dispense with semiconductor photodetecting devices. But the present phtodetectlng devices still suffer from the difficulty above-mentioned.
The necessity of a sophisticated converging optics, e.g. non-spherical lens or selfoc lens impedes wide prevalence of the optoelectronic telecommunication system. One purpose of this invention is to provide a semiconductor photodetector which dispenses with sophisticated, expensive converging optics. Another purpose of this invention is to provide a photodetector which can prevent extra, outer beams from entering the non-receiving region. The other purpose of this invention to provide a semiconductor detector which enables us to construct a high speed digital optoelectronic telecommunication system.